The field of the invention pertains to truck-trailers and in particular to multi-trailer vehicles commonly known as "doubles" consisting of a tractor, semi-trailer and full trailer or "triples" consisting of a tractor, semi-trailer and two full trailers. With such an arrangement, the first trailer is towed by a fifth wheel mounted on the tractor and each following trailer is a semi-trailer towed by a fifth wheel mounted on a dolly in turn attached by a single point hitch to the back of the first trailer. The most common such vehicle in use in the United States is the "Western Doubles", where both trailers are about 28 feet in length.
The most common and widely used dolly is the so-called A-Dolly, which hitches to the towing vehicle or first trailer using a single point hitch, commonly called a pintle hitch. The A-Dolly provides "wagon tongue" steering for the second trailer by allowing the entire dolly to steer relative to its semi-trailer about the fifth wheel vertical axis on the dolly as well as relative to the towing trailer about the pintle hitch vertical axis. The dolly tires do not, however, steer relative to the dolly frame. Recently, steerable A-Dollies have been developed as disclosed in U.S. Pat. No. 4,451,058.
Commercial vehicles of either truck and full trailer or multi-trailer configurations which employ the above common A-Dollies generally possess undesirable characteristics such as rearward amplification. Rearward amplification, sometimes described as a "crack-the-whip" phenomenon, implies that in rapid evasive maneuvers such as emergency lane changes, the rearward elements such as the second trailer and dolly of the vehicle train experience motions which are substantially amplified compared to the motions of the towing tractor and first trailer. Rearward amplification is known to be the basic cause of many accidents in which roll over of the last trailer or second trailer occurs while the remaining elements of the vehicle remain unscathed. As a result of recent changes in federal road usage laws, the use of multi-trailer vehicles is expected to grow rapidly in the next several years so that concern over rearward amplification and its consequences is expected to increase.
A second general class of dollies known as B-Dollies represent an improvement over A-Dollies. B-Dollies are generally characterized by a double tow bar arrangement which eliminates steering of the dolly with respect to the towing vehicle, most commonly the first trailer. The double hitch also couples the towing vehicle and the dolly and second trailer in roll. The elimination of pintle hitch steering tends to reduce rearward amplification. Coupling in roll further assists in preventing rollover of either of the vehicle trailers.
As an example, B-Dollies can be made in which the dolly tires do not steer relative to the dolly frame. Such a configuration has been found in many cases to lead to unacceptable levels of frame stress and of tire scuffing and wear. Also, difficulties arise in low speed maneuvering. "Steering B-Dollies" in which the dolly tires are allowed to steer by castering mechanisms reduce this problem. However, dolly tires cannot be allowed to steer completely free or undesirable dynamic performance results. The success of a self-steering B-Dolly depends on the compromise between the need for high steering resistance or centering action for good dynamic performance and the need for low steering resistance or centering action for obtaining low levels of stress, scuffing and wear. Such a dolly is disclosed in U.S. Pat. No. 4,162,082. A good compromise is generally quite difficult to attain and the long-term reliability of self-steering B-Dolly mechanisms is of particular concern since degradation of the desired steering resistance over time could lead to dangerous dynamic performance.
Ackerman geometry is well known as the most desirable case for low speed maneuvering since it minimizes tire scuffing, wear and structural stress. With Ackerman geometry all tires of the dolly and the tires on the rear axles of both the first and second trailers are aligned tangent to circular paths which all have the same turn center. An example of Ackerman geometry as applied to the steering of the rear tandem wheels of a semi-trailer is disclosed in U.S. Pat. No. 2,342,697.